1. Field of the Invention
The present invention concerns an implantable medical lead for tissue stimulation of the type that is adapted to be attached to an implantable tissue stimulator that includes a pulse generator, the electrode lead being of the type having at least two stimulation electrodes to apply stimulation pulses to the tissue, with the stimulation electrodes being located close to the distal end of the electrode lead, and the electrode lead having at least two electrical conductors that electrically connect the electrodes to the pulse generator.
2. Description of the Prior Art
Implantable pulse generator (IPG) systems are considered contraindicative to Magnetic Resonance Imaging (MRI). One concern discussed regarding compatibility of IPG systems and MRI scanning is heating at or close to the lead tip, caused by currents in the lead induced by the applied RF-field from the MRI system, i.e. the IPG lead is acting as an antenna picking up the RF field during the MRI scan.
If the heating is too high, there is a concern that there may be damages to the cardiac tissue.
The use of MRI scans for diagnostics is growing extensively and an increasing, already large number of IPG patients would benefit from MRI scans. It is thus desirable to reduce any heating at or close to the lead tip to acceptable and safe levels.
In the prior art a number of patents and patent applications exist related to different solutions of the above problem with MRI-scanning of IPG patients.
The solutions proposed in the prior art may be divided into two main groups.
The first group is essentially based upon filtering, insulating or compensation techniques to reduce effects of MRI.
U.S. Pat. No. 7,363,090, for example, includes a band stop filter arranged to attenuate a current flow through the lead wire along a range of selected frequencies.
In U.S. Pat. No. 7,123,013 a tuneable compensation circuit is connected to the lead wire line. This circuit applies supplemental impedance to the wire line to cause the characteristic impedance of the wire line to become unbalanced, thereby reducing the effects of induced voltages caused by the MRI field.
In US-2003/0204217 an electrode isolation system electrically isolates the lead electrodes from the voltage discharge unit during time intervals between the voltage pulses.
In US-2007/0238975 an MRI gradient magnetic field is sensed and the system switches from a first electrical signal processing mode to a second electrical signal processing mode based upon the sensed field.
US-2008/0079429 relates to an implantable medical device with two medical leads and a filter circuit coupled to the distal end of the first lead. A compensation circuit provides compensation voltage to enable the filter to effectively block changing magnetic field induced current in the second lead from passing through a second electrode of the distal end of the second lead.
In the second group, represented by US-2002/0116028, so-called photonic leads or catheters are used where electrical pulses output by the pulse generator are converted into light energy and directed into the proximal end of the photonic catheter. The photonic catheter includes an optical conduction pathway and light entering the proximal end of the catheter is transmitted through the optical pathway, where it is collected and converted back to electrical energy at the distal end of the photonic catheter. The optical pulses are then converted to electrical pulses and delivered to the heart electrodes.
In US-2002/0116029 a similar system is disclosed, differing in that a miniature pulse generator is arranged at the distal end of the photonic catheter that stores electrical energy received via the optical conductors and periodically releases that energy to deliver electrical pulses to the bipolar heart electrodes. A similar device is also disclosed in US-2002/0116034.